The preparation of pure compounds of the formula (I) with acceptable yields by the Williamson Synthesis failed heretofore because of the formation of azeotropic mixtures of (I) with the starting materials of the formula (II). Despite large expenditures of distillation energy, which is of disadvantage, small amounts of pure (I) in addition to large amounts of mixtures of (I) and (II) which are unsuitable for further processing have always been obtained. Attempts to form the alcoholate completely without the use of an excess of (II) and to avoid the occurrence of the azeotrope remained unsuccessful. Very long reaction times lead, moreover, to side reactions and losses in yield. The starting compound (II) could not be reacted completely according to the prior art methods.